Sensors using analog technology for determining the level of fuel in a vehicle fuel tank have long been known. These sensors may employ a variation in resistance, capacitance, etc. with fuel level to develop an analog output signal having a magnitude proportional to the level of fuel in the tank. The signal may be applied to the coil of a visual fuel level indicator to displace a pointer over a dial or it may be converted to a digital value for driving a numeric or a bar graph display. In the case where a variation in resistance is employed to develop the output signal, the variation in resistance may be obtained by using a float element which floats on the surface of the fuel and selectively closes switches to change the resistance in the sensor circuit as the fuel level changes. A typical fuel gauge of this type is disclosed in U.S. Pat. No. 5,532,673.
More recently, sensors employing digital technology have been developed. These sensors have no moving parts and produce a cyclic pulse width modulated signal indicating the level of fuel in the tank. The digital type sensor will function with a dial and pointer type indicator because the pointer driving coil of the indicator averages the pulse width modulated signal. However, the digital sensor and analog sensor output signals must be processed differently in order to develop an output signal for driving a numeric or a bar graph display.
Since the pulse type sensors have no moving parts and are generally more reliable than sensors employing analog technology, it is desirable to use them on newly produced vehicles and as replacement sensors on older vehicles. Also, it is desirable that the two types of sensors be interchangeable so that, in the event of sensor failure, either type may be used as a replacement part depending on availability. These options are available only if the controller which processes the signals for numeric or bar graph display is capable of automatically recognizing which type of sensor is currently installed on a vehicle.